1. Field of the Invention
The present invention relates to capacitors having a porous electrode body coated with a dielectric metal oxide film and a layer of a material which emits oxygen, and particularly to an improved glass-metal outer layer for capacitors.
2. Description of the Prior Art
Normally such capacitors consist of a porous body of pressed or sintered particles of a film-forming metal, such as tantalum, in which there is embedded a lead-in wire of the same metal. Subsequently, the surface of this electrode body is subjected to anodic oxidation by means of conventional processes. The resultant oxide film which, when using tantalum, consists of tantalum-oxide (Ta.sub.2 O.sub.5), serves as the dielectric of the capacitor. Since the oxide film easily becomes cracked, it is coated with a further layer, usually of manganese dioxide (MnO.sub.2), having the property of emitting oxygen in response to passage of a strong electric current and the heat developed from this current. The oxygen preferably deposits at the fault point and oxidizes the surface thereof.
The manganese dioxide layer also forms the second electrode of the capacitor. Due to the relatively poor conductivity of manganese dioxide, it is necessary to make this layer as thin as possible and to coat it with another layer of improved conductivity. Usually this is a layer of fine carbon particles applied by dipping the electrode body into a colloidal graphite suspension.
Graphite has been used as an electrode coating because the suspension of the colloidal graphite penetrates into the pores of the electrode body coated with the layer of manganese dioxide. Since carbon is not solderable, a solderable coating is usually deposited onto the carbon. This coating consists of an organic binding agent mixed with fine silver particles. This mixture is used because the coating can be deposited in an economical dipping process. Subsequently, this silver layer is provided with a coating of solder metal to which the capacitor lead can be soldered.
The mixture of organic binding agent and silver has good electrical properties, but is very sensitive to increased temperatures. It has proved that the silver of the coating, at higher temperatures, alloys with the deposited solder metal. In addition, due to the solubility of silver in the solder metal, the silver is dissolved out of the organic binding agent, thus causing the binding agent to form a layer of poor conductivity. A further disadvantage is due to the fact that the silver also penetrates into the graphite coating and into the layer of manganese dioxide positioned therebeneath, so that these layers have to be relatively thick.
Up to now, the aforementioned disadvantages have been avoided by taking great care in depositing the layer of solder metal. The use of higher melting temperature metals and the time during which the silver coating may be exposed to higher temperatures, therefore, are subject to narrow tolerances. Besides using a coating of silver in an organic binding agent, other metal coatings are known which are deposited by way of spraying, electroplating or by melting-on suitable metals, such as copper, silver or gold.
In depositing the metal coating by means of the sputtering or spraying method, the graphite layer as well as the manganese-dioxide layer positioned therebeneath, must be relatively thick, because the metal drops hit the electrode body at a considerable speed. As a result, the thin electric layer can easily be punctured and cause short circuits.